panfrost: Precompute scoreboard dependents

panfrost: Precompute scoreboard dependents

Mali job dependency graphs, at least for GLES3.0, have the special
property that a given node will only have at most a single dependent.
This allows us to efficiently precompute the dependent array and
replace an inner loop's O(N) search with an O(1) lookup, bringing the
algorithmic complexity of scoreboarding from O(N^2) to O(N).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>